What Is Listeria?

Listeria—or Listeria monocytogenes, if you’re feeling scholarly—can be found in everything from soil to water, chickens to cows, so it's no surprise when listeria finds its way into the food we eat. In fact, one in 20 adults carries the bacteria but doesn’t experience any symptoms.

Listeria (and the subsequent infection, listeriosis) works like many other foodborne illnesses. “The difference is that listeria can cause life-threatening conditions like meningitis in certain immune-compromised hosts,” says Nancy Simpkins, M.D., an internist at St. Barnabas Medical Center.

Much like E. coli and salmonella infections, listeriosis tends to hit populations with weaker-than-usual immune systems. Adults over the age of 65, pregnant women, newborns, and anyone with an already weak immune system (think those with serious chronic medical conditions such as cancer, diabetes, or HIV, or those undergoing immunosuppressive therapies such as radiation or steroids) are at greater risk of contracting the disease. But it’s how it infects, not who, that makes listeria different.

When listeriosis hits, you might experience typical food poisoning symptoms (stomach cramps, nausea, or diarrhea) a few days after consuming contaminated food, but it takes about two months for the disease to show its true colors. That’s how long the infection needs to multiply in your cells and reach your nervous system, where it erupts with symptoms like headaches, a stiff neck, confusion or changes in your levels of alertness, balance problems, and convulsions. Severe listeriosis can cause bacterial meningitis in adultsListeria monocytogenes meningitis in an immunocompromised patient. Barocci S, Mancini A, Canovari B. The new microbiologica, 2015, Jan.;38(1):1121-7138. and is particularly devastating for newborns and pregnant women, as the infection can pass from mother to fetusListeriosis in Pregnancy: Diagnosis, Treatment, and Prevention. Janakiraman V, Review of Obstretrics and Gynecology. 2008 Fall; 1(4): 179&ndash;185. .

Why Now?

The recent listeria hysteria might make it seem like a newfangled, highly evolved bacteria that’s just now starting to conquer the world, one artisanal ice cream brand at a time, but it's not new. Researchers have observed listeriosis in more than 50 species over the last 90 yearsListeriosis--history and actual developments. Seeliger HP. Infection, 1988, Oct.;16 Suppl 2():0300-8126.. But it wasn’t until the 1980s that a bad batch of coleslaw tipped off scientists to listeria being a food safety issueHistory and epidemiology of listeriosis. Hof H. FEMS immunology and medical microbiology, 2003, Jun.;35(3):0928-8244.. Everything from soft cheese to raw milk, stone fruit to cabbage, mussels to prosciutto has been contaminated by listeria at one point or another.

Part of the reason these recalls seem more common today is the food industry is becoming more transparent and vigilant about testing, so we hear about outbreaks more frequently, says Kantha Shelke, Ph.D., a food scientist at the Institute of Food Technologists. The aggressive production schedules at many food producers also means that sanitation might not always be up to snuff (um, yuck!), she says.

And then there’s our desire for food to have a long shelf life. “We are really pushing the boundaries of nature by subjecting foods that are typically eaten within a very short time of their preparation to long distribution and storage cycles, all of which foster the growth of listeria,” Shelke says.

The Takeaway

First, the bad news: There is no definitive defense against listeria. The good news: That means you can and should keep on living your life, albeit with some precautions.

“There is little an average cantaloupe-eating, ice cream cone-ordering consumer can do besides adhering to proper food preparation hygiene and buying from manufacturers and retailers who have high standards of cleanliness,” Shelke says.

Ideally, you should shop locally and frequently to maximize your food’s freshness and minimize the possibility of it coming into contact with contaminated surfaces. When you get the goods home, eat them within a few days of purchase. Why? Because unlike E. coli and salmonella, listeria flourishes in the fridge, Shelke says. To kill it, you need to cook it (and avoid raw dairy—pasteurization kills listeria as well). If you’re eating fresh produce, even if it’s from your own garden, don’t skimp on food prep. Clean fruit and vegetables thoroughly with a produce brush, Simpkins says, ideally one you can toss in the dishwasher to kill remaining bacteria.

You should also be on the lookout for food recalls. If you feel ill after eating a product that has been recalled, or if you experience food poisoning symptoms after eating food that is a well-known host for listeria (like deli meats or unpasteurized dairy), dial your doc. If the nervous system symptoms hit you, go straight to your local ER. If your blood tests positive for the bacteria, you’ll be given antibiotics which should clear up the infection. If you test negative but remain highly symptomatic, you might not be out of the woods. Doctors may opt for further testing.

Even with all of these warnings and precautions, there is no need to panic. Most people never contract listeriosis—the CDC reports an estimated 1,600 cases per year, which accounts for about 1 percent of all microbial food borne disease, says James Hardeman, M.D., a pulmonary and critical care physician. And if you do, antibiotics can really do the trick. Just, you know, be sensible. And maybe opt for DIY ice cream this summer.